Solvent-free production method for producing acrylate pressure-sensitive adhesive substances

ABSTRACT

Method for the continuous polymerization of acrylic monomers to polyacrylates in an extruder and in the presence of polymerization regulators

The present invention relates to an improved process for the continuous preparation of acrylate pressure-sensitive adhesives by solvent-free polymerization.

For industrial pressure-sensitive adhesive tape applications it is very common to use polyacrylate pressure-sensitive adhesives. Polyacrylates possess a variety of advantages over other elastomers. They are highly stable toward UV light, oxygen, and ozone. Synthetic and natural rubber adhesives generally contain double bonds, which makes these adhesives unstable to the aforementioned environmental influences. Another advantage of polyacrylates is their transparency and their usefulness across a relatively wide temperature range.

Polyacrylate pressure-sensitive adhesives are generally prepared in solution by a free-radical polymerization. The polyacrylates are generally coated onto the corresponding backing material from solution, using a coating bar, and then dried. In order to increase the cohesion the polymer is crosslinked. Curing proceeds thermally or by UV crosslinking or by EB curing (EB: electron beams). The operation described is relatively costly and environmentally objectionable, since as a general rule the solvent is not recycled and the high consumption of organic solvents represents a high environmental burden.

It is very difficult, moreover, to produce pressure-sensitive adhesive tapes at high coatweight without bubbles.

One remedy for these disadvantages is the hotmelt process. In this process the pressure-sensitive adhesive (PSA) is applied from the melt to the backing material. This new technology, however, has its limitations. Prior to coating, the solvent is removed from the PSA, which is still prepared in solution, in a drying extruder. This concentration procedure, as it is known, in the drying extruder removes the solvent from the polymer solution down to a residual level of <2%. Since polymerization therefore continues to take place in solution, the high consumption of organic solvents represents a problem both environmentally and economically. A further factor is that possible solvent residues in the adhesive can lead to odor nuisance in the course of subsequent use.

A solvent-free polymerization of the acrylate PSA, therefore, would result in a considerable improvement of the process as a whole. This, however, is very difficult, since polymerizations are associated with considerable heat production and an increase in viscosity. The high viscosities can lead to problems of mixing and hence also of heat removal and reaction regime. The free-radical polymerization of vinyl monomers is known and extensively described (Ullmann's Encyclopedia of Industrial Chemistry, 2nd Edt. Vol. A21, 1992, 305ff, VCH Weinheim).

EP 016 03 94 describes the solvent-free preparation of polyacrylates in a twin-screw extruder. The acrylate hotmelt PSAs prepared by that process, however, have a gel fraction which is in some cases considerably high, of up to 55%, thereby severely impairing the further processing of the PSAs. The high gel fraction means that the adhesive can no longer be coated.

One solution for reducing this disadvantage is offered by polyacrylate adhesives with a low average molecular weight and narrow molecular weight distribution. Reducing the low-molecular-weight fraction lowers the number of oligomers which reduce the shear strength of the PSA.

A variety of polymerization methods are suitable for preparing low-molecular-weight PSAs. State of the art is the use of regulators, such as of alcohols or thiols (MakromolekOle, Hans-Georg Elias, 5th edition, 1990, Hüthig & Wepf Verlag Basle). These regulators reduce the molecular weight but broaden the molecular weight distribution.

A further controlled polymerization method employed is that of atom transfer radical polymerization, ATRP, where the initiators used are preferably, monofunctional or difunctional, secondary or tertiary halides and the halide(s) is(are) abstracted using complexes of Cu, of Ni, of Fe, of Pd, of Pt, of Ru, of Os, of Rh, of Co, of Ir, of Cu, of Ag or of Au [EP 0 824 111; EP 0 826 698; EP 0 824 110; EP 0 841 346; EP 0 850 957]. The various possibilities of ATRP are further described in U.S. Pat. No. 5,945,491, U.S. Pat. No. 5,854,364, and U.S. Pat. No. 5,789,487. Generally speaking, metal catalysts are used, a side effect of which is to affect adversely the aging of the PSAs (gelling, transesterification). Furthermore, the majority of metal catalysts are toxic, discolor the adhesive, and are removable from the polymer only by means of costly and inconvenient precipitation procedures.

Another version is the RAFT process (reversible addition-fragmentation chain transfer). The process is described exhaustively in WO 98/01478 and WO 99/31144, but in the manner depicted there is unsuitable for preparing PSAs, since the conversions achieved are very low and the average molecular weight of the polymers prepared is too low for acrylate PSAs. The polymers described cannot, therefore, be used as acrylate PSAs. An improvement was achieved with the process described by BDF in DE 100 30 217.

U.S. Pat. No. 4,581,429 discloses a controlled free-radical polymerization process. The process employs as its initiator a compound of the formula R′R″N—O—X, in which X represents a free radical species able to polymerize unsaturated monomers. In general, however, the reactions exhibit low conversion rates. A particular problem is the polymerization of acrylates, which proceeds only to very low yields and molecular weights.

WO 98/13392 describes open-chain alkoxyamine compounds which have a symmetrical substitution pattern. EP 0 735 052 A1 discloses a process for preparing thermoplastic polymers having narrow polydispersities.

WO 96/24620 describes a polymerization process for which very special radical compounds are described, such as phosphorus-containing nitroxides, for example.

WO 98/30601 discloses specific nitroxyls based on imidazolidine.

WO 98/4408 discloses specific nitroxyls based on morpholines, piperazinones, and piperazinediones.

DE 199 49 352 A1 discloses heterocyclic alkoxyamines as regulators in controlled free-radical polymerizations.

Corresponding developments of the alkoxyamines or of the corresponding free nitroxides improved the efficiency for the preparation of polyacrylates [Hawker, C. J., paper, National Meeting of the American Chemical Society, San Francisco, Spring 1997; Husemann, M., IUPAC World Polymer Meeting 1998, Gold Coast, Australia, paper on “Novel Approaches to Polymeric Brushes using ‘Living’ Free Radical Polymerizations” (July 1998)].

The aforementioned patents and papers attempted to improve the control of free-radical polymerization reactions. Nevertheless, there exists a need for a polymerization process which is highly reactive and with which high conversions can be realized in conjunction with high molecular weight and low polydispersity. This is so in particular for the copolymerization of acrylate PSAs, since, here, high molecular weights are essential for PSA applications. These requirements were met in DE 100 36 801 A1, where the polymerization takes place in organic solvent or water as solvent, so giving rise, here again, to the problem of the high solvent consumption and/or solvent removal.

It is an object of the invention, therefore, to provide a process for solvent-free preparation of acrylate hotmelt PSAs which exhibits the disadvantages of the cited prior art either not at all or only to a reduced extent.

Surprisingly, it has been found that the use of regulating substances which produce a narrow molecular weight distribution of the polyacrylates is particularly advantageous in its effects on the solvent-free polymerization process in a reaction extruder, in particular a planetary roller extruder.

As a result of the use of substances which regulate the polymerization process and are described in more detail below, solvent-free polymerization in a planetary roller extruder produces polymers having a narrow molecular weight distribution. The fraction of low-molecular-weight and of high-molecular-weight molecules in the polymer is sharply reduced. As a result of the drop in the high molecular weight fractions, the flow viscosity is lower. This leads to improved mixing in the planetary roller extruder and hence also to an improvement in heat input and heat removal. With the use of the regulating substances which result in the polymerization in the planetary roller extruder producing polymers having a narrow molecular weight distribution it has surprisingly been found that, as a result, the process of solvent-free polymerization is considerably less sensitive to operational fluctuations. The tendency to form gel in the case of operational fluctuations in, for example, the temperature or rotational speed, for instance, is considerably and unpredictably reduced. It has been found, moreover, that the polymers thus prepared exhibit a higher crosslinking efficiency, which is advantageous for the adhesive technology properties.

The invention accordingly provides a process for continuous polymerization of acrylic monomers to polyacrylates in the presence of polymerization regulators, at least one polymerization step being carried out within at least one reaction extruder. Very advantageously the reaction extruder is a planetary roller extruder, in particular a hydraulically filled planetary roller extruder.

The polymerization regulators are selected advantageously from the group of nitroxide regulators and/or RAFT regulators, particularly the alkoxyamines, triazolinyl compounds, thioesters and/or thiocarbonates.

Regulators which have proven particularly suitable for solvent-free polymerization in a planetary roller extruder are asymmetric alkoxyamines of type (II) in conjunction with their free nitroxyl precursors and with an azo or peroxo initiator which exhibits slow thermal decomposition.

With great advantage it is possible to use an initiator system for free-radical polymerizations that is composed of a combination of compounds of the general formulae

in which

-   -   R′, R″, R″′ and R″″ are selected independently of one another         and are         -   a) branched and unbranched C₁ to C₁₈ alkyl radicals; C₃ to             C₁₈ alkenyl radicals; C₃ to C₁₈ alkynyl radicals;         -   b) C₃ to C₁₈ alkynyl radicals; C₃ to C₁₈ alkenyl radicals;             C₁ to C₁₈ alkyl radicals substituted by at least one OH             group or halogen atom or silyl ether;     -   c) C₂-C₁₈ heteroalkyl radicals having at least one oxygen atom         and/or NR group in the carbon chain; R being selected from one         of the groups a), b) or d) to g),         -   d) C₃-C₁₈ alkynyl radicals, C₃-C₁₈ alkenyl radicals, Cl-C,₈             alkyl radicals substituted by at least one ester group,             amine group, carbonate group and/or epoxide group and/or by             sulfur and/or by sulfur compounds, especially thioethers or             dithio compounds;         -   e) C₃-C₁₂ cycloalkyl radicals;         -   f) C₆-C₁₀ aryl radicals;         -   g) hydrogen;     -   is a group having at least one carbon atom and is such that the         free radical X- derived from X is able to initiate         polymerization of ethylenically unsaturated monomers.

-   Halogens here are preferably F, Cl, Br or I, more preferably Cl and     Br. As alkyl, alkenyl, and alkynyl radicals in the various     substituents, both linear chains and branched chains are     outstandingly suitable.

-   Examples of alkyl radicals containing 1 to 18 carbon atoms are     methyl, ethyl, propyl, isobutyl, butyl, isobutyl, tert-butyl,     pentyl, 2-pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, tert-octyl,     nonyl, decyl, undecyl, tridecyl, tetradecyl, hexadecyl and     octadecyl.

-   Examples of alkenyl radicals having 3 to 18 carbon atoms are     propenyl, 2-butenyl, 3-butenyl, isobutenyl, n-2,4-pentadienyl,     3-methyl-2-butenyl, n-2-octenyl, n-2-dodecenyl, isododecenyl and     oleyl.

-   Examples of alkynyl having 3 to 18 carbon atoms are propynyl,     2-butynyl, 3-butynyl, n-2-octynyl and n-2-octadecynyl.

-   Examples of hydroxy-substituted alkyl radicals are hydroxypropyl,     hydroxybutyl or hydroxyhexyl.

-   Examples of halogen-substituted alkyl radicals are dichlorobutyl,     monobromobutyl or trichlorohexyl.

-   A suitable C₂-C₁₈ heteroaryl radical having at least one oxygen atom     in the carbon chain is, for example, —CH₂—CH₂—O—CH₂—CH₃.

-   Examples of C₃-C₁₂ cycloalkyl radicals include cyclopropyl,     cyclopentyl, cyclohexyl or trimethylcyclohexyl.

-   Examples of C₆-C₁₀ aryl radicals include phenyl, naphthyl, benzyl,     or further substituted phenyl radicals, such as, for example, ethyl,     toluene, xylene, mesitylene, isopropylbenzene, dichlorobenzene or     bromotoluene.

The lists above serve only as examples of the respective groups of the compound, and do not possess any claim to completeness.

In one particularly preferred embodiment of the invention a combination of the compounds (Ia) and (IIa) is used as initiator system.

In a very advantageous development of the inventive initiator system, additionally, further free-radical initiators for the polymerization are present, especially thermally decomposing, free-radical-forming azo or peroxo initiators. In principle, however, suitability for this purpose is possessed by all customary initiators known for acrylates. The production of C-centered radicals is described in Houben Weyl, Methoden der Organischen Chemie, Vol. E 19a, pp. 60-147. These methods are preferentially employed analogously.

Examples of free-radical sources are peroxides, hydroperoxides, and azo compounds; as a number of nonexclusive examples of typical free-radical initiators, mention may be made here of potassium peroxodisulfate, dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide, di-tert-butyl peroxide, azodiisobutyronitrile, cyclohexylsulfonyl acetyl peroxide, diisopropyl percarbonate, tert-butyl peroctoate, and benzpinacol. In one very preferred version the free-radical initiator used is 1,1′-azobis-(cyclohexanecarbonitrile) (Vazo 88™ from DuPont).

The compounds of the formula (II) are present preferably in an amount of 0.0001 mol % to 1 mol %, more preferably in an amount of 0.0008 to 0.0002 mol %, based on the monomers. The compounds of the formula (I) is present preferably in an amount of 1 mol % to 10 mol %, more preferably in an amount of 3 to 7 mol %, based on compound (II). The thermally decomposing initiator from c) is present with particular preference in an amount of 1 to 10 mol %, more preferably in an amount of 3 to 7 mol %, based on compound of the formula (II).

The reaction is initiated by scission of the X—O bond of the initiator component of the formula (II). The scission of the bond is brought about preferably by ultrasound treatment, heating or exposure to electromagnetic radiation in the wavelength range of y radiation, or by microwaves. More preferably the scission of the C—O bond is brought about by heating and takes place at a temperature between 70 and 160° C.

In one further advantageous development the initiator system used is at least one triazolinyl compound of the general formula

where R^(#), R^(∩∩), R^(###), and R^(####) are chosen independently of one another or are identical and are

-   -   branched and unbranched C₁ to C₁₈ alkyl radicals; C₃ to C₁₈         alkenyl radicals; C₃ to C₁₈ alkynyl radicals;     -   C₁ to C₁₈ alkoxy radicals;     -   C₃ to C₁₈ alkynyl radicals; C₃ to C₁₈ alkenyl radicals; C₁ to         C₁₈ alkyl radicals substituted by at least one OH group or         halogen atom or silyl ether;     -   C₂-C₁₈ heteroalkyl radicals having at least one oxygen atom         and/or R^(####) group in the carbon chain; R^(####) being able         to be any desired organic radical, and, in particular, branched         and unbranched C₁ to C₁₈ alkyl radicals; C₃ to C₁₈ alkenyl         radicals; C₃ to C₁₈ alkynyl radicals,     -   C₃-C₈ alkynyl radicals, C₃-C₁₈ alkenyl radicals, Cl-C,₈ alkyl         radicals substituted by at least one ester group, amine group,         carbonate group, cyano group, isocyano group and/or epoxide         group and/or by sulfur, especially thioethers or dithio         compounds;     -   C₃-C₁₂ cycloalkyl radicals;     -   C₆-C₁₀ aryl radicals;     -   hydrogen.

Control reagents (triazolinyl compounds in the sense of the initiator system depicted above) of type (I) are composed, in a more-preferred version, of the following, further-restricted compounds:

Halogens here are preferably F, Cl, Br or I, more preferably Cl and Br. As alkyl, alkenyl, and alkynyl radicals in the various substituents, both linear chains and branched chains are outstandingly suitable.

Examples of alkyl radicals containing 1 to 18 carbon atoms are methyl, ethyl, propyl, isobutyl, butyl, isobutyl, tert-butyl, pentyl, 2-pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, tert-octyl, nonyl, decyl, undecyl, tridecyl, tetradecyl, hexadecyl and octadecyl.

Examples of alkenyl radicals having 3 to 18 carbon atoms are propenyl, 2-butenyl, 3-butenyl, isobutenyl, n-2,4-pentadienyl, 3-methyl-2-butenyl, n-2-octenyl, n-2-dodecenyl, isododecenyl and oleyl.

Examples of alkynyl having 3 to 18 carbon atoms are propynyl, 2-butynyl, 3-butynyl, n-2-octynyl and n-2-octadecynyl.

Examples of hydroxy-substituted alkyl radicals are hydroxypropyl, hydroxybutyl or hydroxyhexyl.

Examples of halogen-substituted alkyl radicals are dichlorobutyl, monobromobutyl or trichlorohexyl.

A suitable C₂-C₁₈ heteroaryl radical having at least one oxygen atom in the carbon chain is, for example, —CH₂—CH₂—O—CH₂—CH₃.

Examples of C₃-C₁₂ cycloalkyl radicals include cyclopropyl, cyclopentyl, cyclohexyl or trimethylcyclohexyl.

Examples of C₆-C₁₀ aryl radicals include phenyl, naphthyl, benzyl, or further substituted phenyl radicals, such as, for example, ethylbenzene, propylbenzene, p-tert-butylbenzyl, etc., toluene, xylene, mesitylene, isopropylbenzene, dichlorobenzene or bromotoluene.

The lists above serve only as examples of the respective groups of the compound, and do not possess any claim to completeness.

In one particularly advantageous procedure the triazolinyl compounds are selected such that R^(###) and R^(####) are joined to one another in the form of a spiro compound.

With great preference compounds (Ia) and (Ib) are used as control reagents.

The compounds of the initiator system are present preferably in an amount of 0.001 mol % to 10 mol %, preferably in an amount of 0.01 to 1 mol %, based on the monomer mixture.

In a further development of the process the solvent-free polymerization was carried out by virtue of the presence of at least one free-radical initiator with at least one thioester as polymerization regulator.

In a particularly preferred version of the inventive process the thioesters used are compounds of the following general structural formula

where R^(§) and R^(§§) are selected independently of one another and Rs is a radical from one of groups i) to iv) and R^(§§) is a radical from one of groups i) to iii):

-   -   i) C₁-C₁₈ alkyl, C₂-C₁₈ alkenyl, C₂-C₈ alkynyl, each linear or         branched; aryl, phenyl, benzyl, aliphatic and aromatic         heterocycles,     -   ii) —NH₂, —NH—R¹, —NR¹R², —NH—C(O)—R¹, —NR¹—C(O)—R²,         —NH—C(S)—R¹, —NR¹—C(S)—R²,     -   where R¹ and R² are radicals selected independently of one         another from group i),     -   iii) —S—R³, —S—C(S)—R³,     -   where R³ is a radical selected from one of groups i) or ii),     -   iv) —O—R³, —O—C(OR)—R³,     -   where R³ is a radical selected from one of groups i) or ii).

Regulators used, accordingly, are preferably dithioesters and trithiocarbonates. In a further advantageous version of the inventive process the thioester is used with a weight fraction of 0.001% -5%, in particular of 0.025% to 0.25%. Moreover, it is very favorable for the inventive purpose if the molar ratio of free-radical initiator to thioester is in the range from 50:1 and 1:1, in particular between 10:1 and 2:1.

Polymerization regulators which can be used with great advantage in this case for the inventive purpose are trithiocarbonates or dithioesters.

For the polymerization it is preferred to use a control reagent of the general formula:

in which

-   -   R^($) and R^($$) are selected independently of one another or         are the same         -   branched and unbranched C₁ to C₁₈ alkyl radicals; C₃ to C₁₈             alkenyl radicals; C₃ to C₁₈ alkynyl radicals;         -   H or C₁ to C₁₈ alkoxy;         -   C₃ to C₁₈ alkynyl radicals; C₃ to C₁₈ alkenyl radicals; C₁             to C₁₈ alkyl radicals substituted by at least one OH group             or halogen atom or silyl ether;         -   C₂-C₁₈ heteroalkyl radicals having at least one oxygen atom             and/or NR* group in the carbon chain;         -   C₃-C₁₈ alkynyl radicals, C₃-C₁₈ alkenyl radicals, Cl-Cl₈             alkyl radicals substituted by at least one ester group,             amine group, carbonate group, cyano group, isocyano group             and/or epoxide group and/or by sulfur;         -   C₃-C₁₂ cycloalkyl radicals;         -   C₆-C₁₈ aryl or benzyl radicals;         -   hydrogen;

Control reagents of type (I) are composed, in a more-preferred version, of the following compounds:

Halogens here are preferably F, Cl, Br or I, more preferably Cl and Br. As alkyl, alkenyl, and alkynyl radicals in the various substituents, both linear chains and branched chains are outstandingly suitable.

Examples of alkyl radicals containing 1 to 18 carbon atoms are methyl, ethyl, propyl, isobutyl, butyl, isobutyl, tert-butyl, pentyl, 2-pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, tert-octyl, nonyl, decyl, undecyl, tridecyl, tetradecyl, hexadecyl and octadecyl.

Examples of alkenyl radicals having 3 to 18 carbon atoms are propenyl, 2-butenyl, 3-butenyl, isobutenyl, n-2,4-pentadienyl, 3-methyl-2-butenyl, n-2-octenyl, n-2-dodecenyl, isododecenyl and oleyl.

Examples of alkynyl having 3 to 18 carbon atoms are propynyl, 2-butynyl, 3-butynyl, n-2-octynyl and n-2-octadecynyl.

Examples of hydroxy-substituted alkyl radicals are hydroxypropyl, hydroxybutyl or hydroxyhexyl.

Examples of halogen-substituted alkyl radicals are dichlorobutyl, monobromobutyl or trichlorohexyl.

A suitable C₂-C₁₈ heteroaryl radical having at least one oxygen atom in the carbon chain is, for example, —CH₂—CH₂—O—CH₂—CH₃.

Examples of C₃-C₁₂ cycloalkyl radicals include cyclopropyl, cyclopentyl, cyclohexyl or trimethylcyclohexyl.

Examples of C₆-C₁₀ aryl radicals include phenyl, naphthyl, benzyl, 4-tert-butylbenzyl or further substituted phenyl, such as, for example, ethyl, toluene, xylene, mesitylene, isopropylbenzene, dichlorobenzene or bromotoluene.

The lists above serve only as examples of the respective groups of the compound, and do not possess any claim to completeness.

Furthermore, compounds of the following types are also suitable as control reagents

where R^($$$) can comprise the aforementioned radicals R^($) or R^($$), independently of their selection.

In one particularly preferred embodiment of the invention compounds (la) and (lha) are used as control reagents.

In connection with the abovementioned polymerizations which proceed by a controlled-growth free-radical mechanism, it is preferred to use initiator systems which additionally comprise further free-radical initiators for the polymerization, especially thermally decomposing, free-radical-forming azo or peroxo initiators. For this purpose, however, suitability is possessed in principle by all customary initiators that are known for acrylates. The production of C-centered radicals is described in Houben Weyl, Methoden der Organischen Chemie, Vol. E 19a, pp. 60-147. These methods are preferentially employed analogously.

In one very advantageous development of the inventive process, additionally, further free-radical initiators for the polymerization are present, especially thermally decomposing initiators, particularly free-radical-forming azo or peroxo initiators. These initiators are preferably added before or in the course of the polymerization, the addition of the further initiators taking place in at least two process stages.

The invention further provides a process for preparing acrylate pressure-sensitive adhesives, in which a monomer mixture composed of ethylenically unsaturated compounds, particularly of (meth)acrylic acid and/or derivatives thereof, is subjected to free-radical polymerization using the inventive initiator system described.

As the monomer mixture it is preferred to use a mixture composed of acrylic monomers of the general formula

where R^(&)=H or CH₃ and R^(&&)=H or an alkyl chain having 1-20 carbon atoms.

In one advantageous embodiment of the inventive process monomers used include, additionally, vinyl compounds having a fraction of up to 30% by weight, in particular one or more vinyl compounds selected from the following group:

vinyl esters, vinyl halides, vinylidene halides, nitrites of ethylenically unsaturated hydrocarbons.

Examples of vinyl compounds of this kind that may be mentioned here include vinyl acetate, N-vinylformamide, vinylpyridines, acrylamides, acrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, ethyl vinyl ether, vinyl chloride, vinylidene chloride, acrylonitrile, maleic anhydride, styrene, without wishing by dint of this enumeration to impose any unnecessary restriction. Furthermore it is possible to use all additional vinyl compounds which fall within the group set out above, and also all other vinyl compounds which do not fall within the classes of compound specified above.

For the polymerization the monomers are selected such that the resulting polymers can be used as industrially useful PSAs, particularly such that the resulting polymers possess PSA properties in accordance with the “Handbook of Pressure Sensitive Adhesive Technology” by Donatas Satas (van Nostrand, New York 1989). For these applications the static glass transition temperature of the resulting polymer is advantageously below 25° C.

The polymers prepared preferably have an average molecular weight of 50 000 to 600 000 g/mol, more preferably between 100 000 and 500 000 g/mol. The average molecular weight is determined by size exclusion chromatography (SEC) or by matrix-assisted laser desorption/ionization—mass spectrometry (MALDI-MS). Depending on reaction regime, the acrylate PSAs prepared by this process possess a polydispersity M_(w)/M_(n) of <4.5.

It has been found that the solvent-free preparation of a polyacrylate hotmelt PSA is possible with advantage in an extruder. The planetary roller extruder, in particular, has proven suitable for a process of this kind. Polymerization in the planetary roller extruder has the advantage that the tendency to form gel is substantially lower than in the case, for example, of a twin-screw extruder, and particularly when regulators and copolymerizable photoinitiators are used the observed tendency to form gel is particularly low. This produces, as a result, narrow-distribution polyacrylate hotmelt PSAs with very good properties for further processing, which, furthermore, can be crosslinked very efficiently.

Owing to the usually short residence time in the case of polymerizations in a planetary roller extruder, it could not have been foreseen that, when using polymerization regulators during the polymerization in the planetary roller extruder, polyacrylate hotmelt PSAs having good crosslinkability would be prepared.

The low polydispersity leads to advantages in the case of polymerization in the planetary roller extruder, thereby reinforcing the outstanding mixing properties which mark out a planetary roller extruder. Through the use of regulators, polymers of low polydispersity are produced, which has advantageous consequences for solvent-free polymerization. The viscosity, which plays a decisive part particularly in the case of solvent-free polymerization, is brought, as a result of the low polydispersity, into a range which is favorable for solvent-free polymerization. With greater polydispersity the viscosity is likewise increased, thereby reducing the heat removal options and also the mixing action in the reactor. These properties are of critical importance to the reliable implementation of solvent-free polymerizations. Likewise, the positive influence of polydispersity on the viscosity enables a higher conversion and also, as a result, reduces the tendency to form gel, which is in turn important for the use of the adhesive as a hotmelt PSA.

The planetary roller extruder is suitable for this solvent-free polymerization in particular by virtue of its outstanding thermal characteristics and also of the extremely diverse possibilities of temperature control.

The extruder used is preferably operated continuously. Partial recycling of the product stream, referred to as loop operation, may also be advantageous. The most advantageous is to prepare a solvent-free polyacrylate PSA in a hydraulically filled planetary roller extruder. Hydraulic filling simplifies compliance with oxygen-free conditions and also the best-possible utilization of the screw length. Moreover, phase boundaries are avoided; such boundaries can have a disruptive effect on the polymerization process.

The monomers can be metered to the polymerization reactor either individually or as a mixture. Preliminary mixing, especially of the copolymerizable photoinitiator, ensures a uniform distribution of the reaction mixture. In principle, however, mixing in the reactor or by combining different reactant streams in an upstream continuous mixer, which is dynamically operated or which may be a static mixer or a micromixer, is also possible.

The addition of further substances such as, for example, initiators, polymerization regulators, and further monomers to the reactant stream along the screw section of the reactor may be advisable. When using a planetary roller extruder composed of a plurality of roller barrels in series, such additions may take place via bores in the connecting flanges of the roller barrels.

With afterdosing of suitable initiators or initiator mixtures it is possible to achieve high conversions without at the same time, as a result of a high concentration of primary radicals, inducing low molecular weights or instances of polymer gelling.

In one development of the process the polymer, following polymerization in a planetary roller extruder, is removed from residual volatile constituents such as unreacted monomers in a devolatilizing extruder. After determination of their composition, these constituents can be recycled to the reactant stream.

In another development of the process, the polymer, following polymerization and, where necessary, devolatilization and the optional addition of one or more of the additives, the addition being able to take place in the polymerization extruder and/or in a downstream compounding extruder, is advantageously coated from the melt without gel onto a backing (“without gel” denotes compliance with the requirements for coatability of the compositions using the coating apparatus which is commonly used and is familiar to the skilled worker for these purposes, particularly for a coatability distinguished by a uniform (homogeneous) coating pattern without inhomogeneities or streaks when coating takes place through the coating nozzles that are commonly used or through a roller applicator).

Then it is advantageous to crosslink the polymer by means of high-energy radiation and/or thermally; this takes place in particular after the operation of coating onto the backing.

In summary it is possible to construct the following scheme for an advantageous procedure:

-   -   polymerization process of a monomer mixture comprising not only         (meth)acrylic acid-based monomers but also polymerization         regulators,     -   the polymerization taking place in a solvent-free process,     -   which is made possible by the use of a planetary roller         extruder.     -   Through the use of the control reagent, polydispersities of 1.2         to 4.5, in particular up to less than 4, are obtained.     -   The polymerization process may be followed by a devolatilizing         operation.     -   The polymer can be further processed directly. Solvent recycling         is unnecessary.     -   The polymer is coated from the melt without gel, and     -   after coating, it is crosslinked using high-energy radiation         and/or thermally.

For the use of the polyacrylates prepared by the inventive process as pressure-sensitive adhesives (PSAs), the polyacrylates are optimized by optional blending with at least one resin. Tackifying resins for addition which can be used include, without exception, all existing tackifier resins which are described in the literature. Representatives that may be mentioned include the pinene resins, indene resins, and rosins, their disproportionated, hydrogenated, polymerized, and esterified derivatives and salts, the aliphatic and aromatic hydrocarbon resins, terpene resins and terpene-phenolic resins, and also C5, C9, and other hydrocarbon resins. Any desired combinations of these and further resins may be used in order to adjust the properties of the resulting adhesive in accordance with requirements. Generally speaking it is possible to use any resins that are compatible (soluble) with the corresponding polyacrylate; in particular, reference may be made to all aliphatic, aromatic, and alkylaromatic hydrocarbon resins, hydrocarbon resins based on single monomers, hydrogenated hydrocarbon resins, functional hydrocarbon resins, and natural resins. Express reference is made to the depiction of the state of knowledge in the “Handbook of Pressure Sensitive Adhesive Technology”, by Donatas Satas (van Nostrand, 1989).

In a further advantageous development, one or more plasticizers are metered into the PSA, such as low-molecular-weight polyacrylates, phthalates, whale oil plasticizers or plasticizing resins, for example.

The acrylate hotmelts may further be blended with one or more additives such as aging inhibitors, light stabilizers, ozone protectants, fatty acids, resins, nucleators, expandants, compounding agents and/or accelerants.

Additionally they may be admixed with one or more fillers such as fibers, carbon black, zinc oxide, titanium dioxide, solid or hollow glass (micro)beads, microbeads of other materials, silica, silicates, and chalk, the addition of blocking-free isocyanates being a further possibility.

Particularly for PSA use it is of advantage to the inventive process if the polyacrylate is applied as a layer preferably from the melt to a backing or to a backing material.

Then, in one advantageous version of the process, the polyacrylate material is applied as a hotmelt composition in the form of a layer to a backing or to a backing material.

Backing materials used for the PSA, for adhesive tapes for example, are the materials that are customary and familiar to the skilled worker, such as films (polyesters, PET, PE, PP, BOPP, PVC), nonwovens, foams, wovens, and woven sheets, and also release paper (glassine, HDPE, LDPE). This enumeration is not exhaustive.

For the PSA utility it is particularly advantageous to crosslink the polyacrylates after they have been coated onto the backing or onto the backing material. To produce the PSA tapes the above-described polymers are for this purpose optionally blended with crosslinkers. Crosslinking may be induced, advantageously, by thermal means or by means of high-energy radiation, in the latter case in particular by electron beams (EB) or, following the addition of appropriate photoinitiators, by means of ultraviolet radiation.

Examples of preferred substances which crosslink under irradiation in accordance with the inventive process are difunctional or polyfunctional acrylates or difunctional or polyfunctional urethane acrylates, difunctional or polyfunctional isocyanates or difunctional or polyfunctional epoxides. Use may also be made here, however, of all further difunctional or polyfunctional compounds which are familiar to the skilled worker and are capable of crosslinking polyacrylates.

Suitable photoinitiators are preferably Norrish type I and type II cleaving compounds, some possible examples of both classes being benzophenone derivatives, acetophenone derivatives, benzil derivatives, benzoin derivatives, hydroxyalkyphenone derivatives, phenyl cyclohexyl ketone derivatives, anthraquinone derivatives, thioxanthone derivatives, triazine derivatives, or fluorenone derivatives, this enumeration making no claim to completeness.

Also claimed is the use of the polyacrylate prepared by the inventive process as a pressure-sensitive adhesive.

Of particular advantage is the use of the polyacrylate PSA prepared as described for an adhesive tape, in which case the polyacrylate PSA may have been applied to one or both sides of a backing.

EXAMPLES

Practical Implementations

Implementation of the Polymerization (Method A):

The polymerization was implemented using a planetary roller extruder consisting of three roller barrels in series, as the reactor. The roller barrels used have a roller diameter of D=70 mm and were fitted with 7 planetary spindles. Not only the central spindle but also the roller barrels are equipped with temperature-control circuits that are separate from one another. The temperature-control medium used was pressurized water.

For the polymerization the reactor is operated continuously. Before commencement of metering the reactor is flushed with nitrogen for one hour. A mixture is produced from monomers and initiator. Nitrogen is passed through this initial charge in order to render it inert. By means of a pump, the reaction mixture is conveyed through a static mixer, which is equipped with further feed devices, and then through a heat exchanger into the reactor. The reaction mixture is added to the reactor continuously via a bore on the periphery of the first roller barrel. At the exit from the reactor there is a valve which is used to ensure the hydraulic filling of the reactor.

The heat exchanger for feed preheating, central spindle, and roller barrels are controlled to the particular desired temperatures. In the case of the central spindle a temperature of 80° C. was set; the medium for feed preheating to 90° C. Roller barrels 1 and 3 were controlled to 100° C., roller barrel 2 to 95° C.

The rotary speed of the central spindle was 50 revolutions per minute. The hydrodynamic residence time was 15 minutes. Following emergence from the reactor, a sample is taken for determination of the conversion. Subsequently, remaining volatile constituents are removed in a devolatilizing extruder.

Production of Swatch Specimens (Method B):

The adhesive is coated at 50 g/m² onto a Saran-primed PET film 23 μm thick, using a hotmelt coater with two heatable rollers.

Preperation of 2,2′-bisphenylethyl thiocarbonate

The 2,2′-bisphenylethyl thiocarbonate is synthesized starting from 2-phenylethyl bromide with carbon disulfide and sodium hydroxide in accordance with a set of instructions in Synth. Communications 18(13), pp. 1531-6, 1988. Yield after distillation: 72%. Characterization: ¹H NMR (CDCl₃) δ (ppm): 7.20-7.40 (m, 10 H), 1.53, 1.59 (2×d, 6 H), 3.71, 381 (2×m, 2 H).

Test Methods

The following test methods were employed in order to evaluate the properties of the polymers and of the PSAs prepared.

Determination of Conversion (Test A.)

The conversion was determined gravimetrically and is expressed as a percentage in relation to the amount by weight of the monomers used. The polymer is isolated by being dried in a vacuum oven. The weight of the polymer is taken and divided by the initial weight of the monomers employed. The calculated figure corresponds to the percentage conversion.

Gel Permeation Chromatography GPC (Test A)

The average molecular weight M_(w) and the polydispersity PD were determined via gel permeation chromatography. The eluent used was THF with 0.1% by volume trifluoroacetic acid. Measurement took place at 25° C. The precolumn used was PSS-SDV, 5 μ, 10³ A, ID 8.0 mm×50 mm. Separation was carried out using the columns PSS-SDV, 5 μ, 10³ and also 10⁵ and 10⁶ each with ID 8.0 mm×300 mm. The sample concentration was 4 g/l, the flow rate 1.0 ml per minute. Measurement was carried out against PMMA standards.

EXAMPLES Example 1 Broad M_(w) Distribution: High Molar Mass

A polymer was prepared by method A. Components used were 5% acrylic acid, 95% n-butyl acrylate and 0.015% azoisobutyronitrile (AIBN, Vazo 64™, DuPont). The average molecular weight and the polydispersity were determined by means of test B, the conversion by test A, and the gel value by test C. Subsequently a swatch specimen was produced in accordance with method B.

Example 2 Narrow M_(w) Distribution: Low Molar Mass

A polymer was prepared by method A. Components used were 5% acrylic acid, 95% n-butyl acrylate and also 0.124% 2,2,-bisphenylethyl thiocarbonate and 0.015% azoisobutyronitrile (AIBN, Vazo 64™, DuPont). The average molecular weight and the polydispersity were determined by means of test B, the conversion by test A, and the gel value by test C. Subsequently a swatch specimen was produced in accordance with method B.

Example 3 Narrow M_(w) Distribution: Low Molar Mass

A polymer was prepared by method A. Components used were 1 % acrylic acid, 49.5% n-butyl acrylate, 49.5% 2-ethylhexyl acrylate and also 0.124% 2,2,-bisphenylethyl thiocarbonate and 0.015% azoisobutyronitrile (AIBN, Vazo 64™, DuPont). The average molecular weight and the polydispersity were determined by means of test B, the conversion by test A, and the gel value by test C. Subsequently a swatch specimen was produced in accordance with method B.

Results

Table 1 below initially collates the results of the polymerizations: TABLE 1 Required roller M_(w) Polydispersity Conversion temperature for Example [g/mol] PD [%] coating [°] 1 2 380 000   6.1 72 Not coatable 2 557 000 3.5 65 120 3 431 000 3.4 63 110 M_(w): average molecular weight from GPC PD: M_(w)/M_(n) = polydispersity from GPC

Example 1 serves as the reference example. For the inventive process, examples 2 to 3 are added. In examples 2 to 3, acrylate PSAs with a low molar mass were prepared. Through the use of a regulator, polymers having a narrow molecular weight distribution were obtained.

The advantages of the process become clear when considering the coatability of the acrylate material. Example 1 has a very high molecular mass and cannot be coated. As a result of the use of the regulator in the case of examples 2 and 3, the molecular weight is lowered to such an extent that coating, which is necessary for use in an adhesive tape, is possible. Thus example 2, with a M_(w) of 557 000 g/mol, and example 3, with a lower M_(w) of 431 000 g/mol are coatable at 120° C. and at just 110° C. Through the process of the invention it is possible to process the prepared adhesive at a low coating temperature. Accordingly, the adhesive tapes can be produced entirely without solvent. 

1. A process for continuous polymerization of acrylic monomers to polyacrylates in the presence of polymerization regulators, wherin at least one polymerization step is carried out within at least one reaction extruder.
 2. The process of claim 1, wherein the polymerization regulators are selected from the group consisting of nitroxide regulators, RAFT regulators or both.
 3. The process of claim 1, wherein the polyacrylates have a polydispersity D=M_(w)/M_(n) of not more than 4.5.
 4. The process of claim 1, wherein the polyacrylates have weight-average molecular weights of 50 000 to 600 000 g/mol.
 5. The process of claim 1, wherein said at least one polymerization step is carried out as a bulk polymerization.
 6. The process of claim 1, wherein said reaction extruder is a planetary roller extruder.
 7. The process of claim 1, wherein downstream of the screw length of the reaction extruder further substances are added.
 8. The process of claim 1, wherin the polymerization process is followed by devolatilization.
 9. The process of claim 1, wherein, after the polymerization and any subsequent devolatilization, the polyacrylate is coated from the melt, without gel, onto a backing.
 10. The process of claim 1, wherein the polymer is crosslinked by means of high-energy radiation and/or thermally, after coating onto a backing.
 11. The process of claim 1, wherein before and/or during the polymerization thermally decomposing, free-radical-forming initiators are added.
 12. A polyacrylate prepared by a the process of claim
 1. 13. A pressure-sensitive adhesive for a single-sided or double-sided adhesive tape comprising the polvacrylate of claim
 12. 14. The process of claim 2, wherein said polymerization regulators are selected from the group consisting of alkoxyamines, triazolinyl compounds, thioesters and thiocarbonates.
 15. The process of claim 4, wherein said weight-average molecular weights are 100 000 to 500 000 g/mol.
 16. The process of claim 6, wherein said planetary extruder is a hydraulically filled planetary roller/extruder.
 17. The process of claim 7, wherein said further substances are selected from the group consisting of initiators, monomers, copolymerizable photoinitiators, and polymerization regulators,
 18. The process of claim 11, wherein said initiators are selected from the group consisting of azo initiators, peroxo initiators and combinations thereof. 